Since drugs are usually available to humans simultaneously with other reinforcers, an understanding of factors that control choice between drugs and other drug or non-drug reinforcers is important to a complete understanding drug abuse. Indeed, the behavioral transition to virtually exclusive choice of drug over other reinforcers could be considered the defining feature of drug abuse. The experiments described in the present proposal will utilize an animal model of drug abuse, i.v. self-administration by monkeys, to examine behavioral and pharmacological variables that control drug choice. By understanding these variables, we hope to enhance our understanding of factors that determine the transition to uncontrolled drug use. The research should contribute important basic information to the development of novel approaches to the treatment of drug abuse. The research has three Specific Aims, each framed in the context of a model that generally predicts choice for reinforcers other than drugs. Specific Aim 1 is to investigate the interaction between reinforcer magnitude and frequency as determinants of drug choice. Results will be compared to predictions of the matching law, that organisms will apportion behavior according to relative magnitudes of reinforcement. Specific Aim 2 is to investigate delay of reinforcement as a controlling variable in drug choice. Results will be compared to predictions of the hyperbolic delay discounting model and models of "impulsivity" and "self-control." Since most drug abusers have a choice between drug and non-drug reinforcers, Specific Aim 3 is to investigate the nature of the alternative reinforcer to drug as a controlling variable in choice.Behavioral economics predicts that choice is controlled by the relative substitutability of alternative reinforcers that are available. The experiments in each Specific Aim will examine representatives of two classes of abused drugs, psychomotor stimulants (cocaine) and opioids (alfentanil). In addition, to examine the generality of our conclusions to behavior maintained by non-drug reinforcers, we will study behavior maintained by food. The proposed research will help elucidate the drug-organism-environment interaction that determines drug abuse, and may provide a laboratory model of uncontrolled drug use. From the point of view of basic behavioral research, the research will expand our understanding of choice in general. Theories of choice should accommodate drug choice as well as choice involving non-drug reinforcers. Further, an understanding of the variables that control the choice to self-administer a drug is crucial to the refinement of drug abuse treatment.